A technology has been developed in which a thin slab (hereunder occasionally referred to as “slab”) 1 to 10 mm in thickness is continuously cast by a twin drum type continuous caster equipped with a pair of cooling drums (hereunder occasionally referred to as “drums”) or a single drum type continuous caster equipped with one cooling drum.
For example, a twin drum type continuous caster is made up of, as major component members, a pair of cooling drums 1, 1′ installed in close and parallel relation to each other with their axes horizontally directed and rotating in opposite directions to each other and side weirs 2 firmly contacting with both end faces of the cooling drums 1, 1′, as shown in FIG. 1.
A sealed chamber 4 is provided above a molten steel pool 3 formed by the cooling drums 1, 1′ and side weirs 2, and an inert gas is supplied to the interior of the sealed chamber 4. When molten steel is continuously supplied from a tundish 5 to the molten steel pool 3, the molten steel solidifies along its parts in contact with the cooling drums 1, 1′ to form solidifying shells. The solidifying shells move down with the rotation of the cooling drums 1, 1′ and are pressure-bonded to each other at a kissing point 6 to form a thin slab C.
As the cooling drums 1, 1′ are used for cooling molten steel during their rotation to produce solidifying shells, they are usually formed of Cu, or a Cu alloy of high thermal conductivity. The cooling drums 1, 1′ keep direct contact with molten steel while forming the molten steel pool 3, but they are out of contact with the molten steel after they pass the kissing point 6 until they again form the molten steel pool 3. Thus, they are sometimes heated by heat held by the molten steel and sometimes cooled by cooling water within the cooling drums 1, 1′ and by the air.
The cooling drums 1, 1′ repeatedly receive a frictional force caused by a relative slip between the thin slab C and the surfaces of the cooling drums 1, 1′ when they pressure-bond the solidifying shells together to form the thin slab C. Therefore, in the event that the surface layers of the cooling drums 1, 1′ are made of Cu or Cu alloy, the peripheral surface layers d are heavily worn away with the progress of casting and do not maintain their surface shape, thus becoming unable to perform casting at an early stage.
With the purpose of preventing such early wear of the surface layer of a drum, a drum structure is known which has a Ni plated layer about 1 mm thick formed on the surface of a cooling drum.
In the event that continuous casting is performed by using cooling drums having the drum structure stated above, there occurs unevenness in a gas gap due to unevenness in adhesion of molten steel to the drums, unevenness in the starting position of solidification due to turbulence in the surface of molten steel, or unevenness in deposited substances on the drum surfaces. As a result, a problem occurs that solidification becomes uneven to cause cracks that impair slab quality.
As this technology is used for producing a thin slab having a shape and thickness close to those of a final product, this technology is indispensably required to make it possible to produce a thin slab completely free from surface defects such as cracks and crevices in order to finally obtain a final product having a required level of quality at a high yield rate.
As a sheet product of stainless steel, in particular, is required to have a high-quality surface appearance, it is a major challenge to cast a thin slab without pickling unevenness.
It is known that the surface defects stated above are formed based on unequal heat contraction stresses developed owing to unevenness in the formation of solidifying shells on the surfaces of the cooling drums, that is, owing to unevenness in the manner in which molten steel solidifies by being quickly cooled, in the course of thin slab casting. Until now, a variety of peripheral surface structures and/or peripheral surface materials for cooling drums have been suggested for cooling and solidifying molten steel in such a manner that unequal heat contraction stresses remaining in the interior of a slab are reduced to the utmost.
For example, a technology is disclosed, by Japanese Unexamined Patent Publication No. S60-184449, in which a Ni plated layer formed on the peripheral surface of a cooling drum is provided with a large number of dimples by shot blasting, photoetching, laser processing or the like, in order to prevent the generation of surface cracks. According to the technology stated above, gas gaps acting as heat insulating layers are formed by these dimples between the cooling drum and a solidifying shell to cause molten steel to be slowly cooled and, also, transferred humps are formed on the surface of a slab by letting the molten steel get into the dimples to an appropriate extent to cause its solidification to start from the peripheries of the transferred humps, thereby equalizing the thickness of the solidifying shell.
Also, a method is disclosed, by Japanese Examined Patent Publication No. H4-33537, wherein a large number of circular or oval dimples are formed on the peripheral surface of a cooling drum, a method is disclosed, by Japanese Unexamined Patent Publication No. H3-174956, wherein the peripheral surface of a cooling drum is roughened by knurling or sandblasting, and a method is disclosed, by Japanese Unexamined Patent Publication No. H9-136145, wherein dimples are formed so as to satisfy maximum diameter≦average diameter+0.30 mm on the peripheral surface of a cooling drum by shot blasting. In any of these methods, an air layer is introduced between a cooling drum and molten steel by forming a large number of dimples or humps on the peripheral surface of a cooling drum, the effective contact area of the peripheral surface of the cooling drum with the molten steel is thereby reduced to relax the cooling of a solidifying shell, and stresses due to heat contraction are relieved to prevent cracks and crevices from being generated due to quick cooling, thus aiming to obtain a thin slab of sound surface appearance.
When either of the methods disclosed by the Japanese Examined Patent Publication No. H4-33537 and by the Japanese Unexamined Patent Publication No. H3-174956 is used, however, molten steel is inserted into dimples formed on the peripheral surface of a cooling drum to form humps on the surface of a slab, and therefore rolling defects such as rolled-in scales and linear scabs are generated in a stage of processing such as rolling in the subsequent processes. In the case of the cooling drum described in the Japanese Unexamined Patent Publication No. H9-136145, dimples of 0.5 to 2.0 mm in diameter, 30 to 70% in area ratio, 60 μm or more in averaged depth, and 100 mm or less in maximum depth are given to the drum by shotblasting, but actually, fine surface defects are still generated on a slab. As the reason for this, it is considered that the distances between adjoining dimples are made excessively large in the stage of shot blasting for forming dimples of the size stated above, their contact surface areas with molten steel are made excessively large because these portions have the shape of a trapezoid, and therefore excessively-cooled portions and slow-cooled portions together exist in a solidifying shell when it is formed, thus generating slab cracks.
As a cooling drum to cope with such a problem, Japanese Unexamined Patent Publication No. H4-238651 discloses a cooling drum wherein dimples 50 to 200 μm in depth are formed with an area ratio of 15 to 30% and, along with this, dimples 10 to 50 μm in depth are formed with an area ratio of 40 to 60% on the peripheral surface of the cooling drum. Further, Japanese Unexamined Patent Publication No. H6-328204 discloses a cooling drum wherein dimples 100 to 300 μm in diameter and 100 to 500 μm in depth are formed with an area ratio of 15 to 50% and, along with this, dimples 400 to 1,000 μm in diameter and 10 to 100 μm in depth are formed with an area ratio of 40 to 60% so that each of the dimple side faces makes an angle of 45° to 75° with a line perpendicular to a peripheral surface tangent on the peripheral surface of the cooling drum.
These cooling drums can suppress the generation of surface cracks and crevices on the surface of a slab while they can suppress the generation of pickling unevenness, the other typical surface defect, and therefore they produce a noticeable effect on the production of a stainless steel sheet product without uneven luster.
Further, Japanese Unexamined Patent Publication No. H11-179494 discloses a cooling drum wherein a large number of humps (preferably, 20 μm or more in height, 0.2 to 1.0 mm in diameter, and 0.2 to 1.0 mm in shortest distance between them) are formed on the peripheral surface of the drum by a means such as photoetching or laser material processing. This cooling drum can suppress surface defects to an extent of nearly zero.
With respect to the cooling drums stated above, however, nothing is specified on the quality of material used for the surface of the cooling drums.
It is apparent that the quality of material used for the surface of a cooling drum affects the surface appearance of a thin slab.
As stated above, a Ni plated layer is usually assumed to be a material for the peripheral surface layer (d in FIG. 1) of a cooling drum. Since the Ni plated layer has lower thermal conductivity than that of a drum base material (Cu, Cu alloy) and a satisfactory bonding property to the drum base material, it is less liable to generate crevices or flakes. Also, it has higher hardness than the base material has and is relatively excellent in abrasion resistance and deformation resistance. However, it is not provided with abrasion resistance or deformation resistance on the level that stably maintains the surface shape of the drum for a long time in actual casting. It has been ascertained that the shape of the peripheral surface layer of a cooling drum changes when it is continuously used for a long time and the change in the shape can become the primary factor of surface cracks on a thin slab.
In view of this, as a cooling drum solving the problem stated above, Japanese Unexamined Patent Publication No. H9-103849 discloses a cooling drum wherein a Ni layer and a Co layer 10 to 500 μm in thickness are formed in this order on the peripheral surface of the drum, the sum of thicknesses of the Ni layer and Co layer being 500 μm to 2 mm, with dimples 30 to 150 μm in average depth formed on the surface of the Co layer. Also, Japanese Unexamined Patent Publication No. H9-103850 discloses a cooling drum wherein a Ni layer is formed on the peripheral surface of the drum, dimples 10 to 50 μm in average depth are provided on the Ni layer by shot blasting, and then an electroplated layer 10 to 500 μm in thickness is provided thereon, thereby causing the average depth of the dimples to be 30 to 150 μm.
These cooling drums are aimed at suppressing the generation of cracks on a thin slab and extending the service life of the drums by improving and devising the peripheral surface structure and peripheral surface material quality of the drums, and they show a noticeable effect.
As stated above, with respect to technologies for continuously casting a thin slab 1 to 10 mm in plate thickness, great success has been achieved in suppressing surface defects including pickling unevenness by improving and devising the peripheral surface structure and/or peripheral surface material quality of a cooling drum.
In operation, however, it is unavoidable that a considerable amount of scum floats and coagulates on the surface of molten steel because of inclusions or mixed-in slag floating up from within the molten steel, even if the generation of scum is suppressed to the greatest possible extent by covering, with an inert atmosphere, a molten steel pool formed by cooling drums and side weirs contacting with both sides thereof for accepting molten steel therein (see the sealed chamber 4 in FIG. 1). When the scum is entrapped between the cooling drums and the molten steel, pickling unevenness appears on a surface of a thin slab.
The portion of such pickling unevenness appears as “uneven luster” on a final sheet product, thus lowering its value as material for a product. Therefore, in order to further enhance the quality and yield rate of a final sheet product, in addition to the suppression of scum generation, it is necessary to take some measures that can inhibit pickling unevenness from being generated on a thin slab even if scum entrapment happens when the thin slab is continuously cast, and if possible, that can eradicate the generation thereof.
In order to find such measures, the present inventors made a close examination into thin slabs on which pickling unevenness appeared. As a result, it was discovered that “a crack” in a form different from the already known “surface crack” was generated in the proximity of a boundary between an area where “pickling unevenness” appeared and an area without it. This “crack” (hereunder referred to as “pickling-unevenness accompanying crack”) is shown in FIG. 2.
As is apparent from FIG. 2, the “pickling-unevenness accompanying crack” is of a nature different, as a matter of course, in origin, position, form and the like from the “surface crack” (hereunder occasionally referred to as “dimple crack”) generated on a portion where no pickling unevenness is generated.
Accordingly, it is difficult to prevent the generation of the “pickling-unevenness accompanying crack” of a different nature as stated above by using conventional means.
As described above, in addition to the task of suppressing the generation of “dimple crack” and “pickling unevenness,” the task of suppressing the generation of “pickling-unevenness accompanying crack” has been newly posed in the continuous casting of a thin slab.
As means for forming dimples on the peripheral surface of a cooling drum, there are shot blasting, photoetching, laser material processing and the like (see Japanese Unexamined Patent Publication No. S60-184449). For an example of laser material processing, Japanese Patent No. 2067959 discloses a method wherein pulsed laser light 0.30 to 1.07 μm in wavelength is used to form holes 500 μm or less in diameter and 50 μm or more in depth, with hole pitches not less than 1.05 times and not more than 5 times the hole diameter. Referring to the example according to this method, four YAG lasers of 500 Hz in pulse repetition frequency are used to form holes with hole pitches of 200 to 250 μm. Assuming that the shape of a cooling drum is of 1 m in diameter and 1 m in width and that holes with pitches of 200 μm are formed on the peripheral surface of the cooling drum, about 80 million holes have to be formed in total. A pulse-light emitting flash lamp is generally used to excite a YAG laser for hole forming and the service life of a flash lamp is 1 to 10 million pulses. Accordingly, even if four YAG lasers are used for hole forming, it is impossible to complete hole forming all over the peripheral surface of the cooling drum within the service life of the flash lamps and therefore the forming work must be stopped to change the lamps.
In such a case, discontinuity of forming appears in portions where the forming is stopped. If a cooling drum having such discontinuity of forming is used in casting, a problem arises that cracks are generated at the discontinuous portions. In this method, if the number of lasers is increased from four, for example, to ten, the problem stated above can be solved. On the other hand, however, a problem arises that an apparatus for forming becomes large-scaled and complicated.
As processing methods using a Q-switched CO2 laser, generally adopted in order to cope with the problems described above, a method of dulling a roll for cold rolling is disclosed by Japanese Patent No. 3027695, and a method of processing a copper alloy by Japanese Unexamined Patent Publication No. H8-309571. In these material processing methods, Q-switched CO2 laser pulses having an initial spike and a pulse tail, with the total pulse width being up to 30 μsec, are used to realize hole forming and the upper limit of hole depth is on the order of 40 μm in any case. Meanwhile, with respect to a cooling drum, it is necessary to form holes, in some cases, 50 μm or more in depth in order to prevent surface cracks and uneven luster. Because of this, there is a problem that the use of the publicly known methods stated above can not realize the hole forming conforming to the expected object of the present invention.
When a metallic material, for example, the peripheral surface of a cooling drum, is processed with laser light for hole forming, a molten substance produced in a boring process is discharged as spatters from holes to the exterior by the vaporizing reaction of the metal itself or by the back pressure of an assist gas and it is often redeposited as dross on the peripheries of the holes. In general, such dross impairs the smoothness of a surface, and hence a means to prevent this is required. In this context, various means of removing or suppressing dross have, so far, been proposed.
A means has been used relatively frequently, up to now, wherein a solid mask layer is provided on the surface of a material to be processed, holes are formed in the material together with the mask, and finally the mask is removed, thereby providing a smooth surface. Since this method requires a process for sticking the mask onto the surface prior to hole forming and a process for removing the mask after laser material processing, it presents, as a whole, problems in terms of work efficiency and cost.
A technique of actively removing dross deposited on a processed surface is disclosed, by Japanese Unexamined Patent Publication No. H10-263855, wherein a “spatula” or a rotary motor-driven grinder is provided adjacent to a processing head for forming fine holes on a work roll for cold rolling as a means for equalizing the distribution of the deposit on the surface of the roll.
Since dross is the deposit of molten substance re-solidified on a processed surface, however, it is difficult to completely remove the dross by using a mechanical means such as “spatula.” Further, in the event that fine holes of the order of 10 to 100 μm in depth are formed, it is difficult to remove only dross by a rotary motor-driven grinder because of its mechanical accuracy, and in some cases, a problem arises that the depth of the holes is decreased by over-grinding. If a method of more actively removing deposited dross is employed, another problem arises that apparatus size is increased by an accessory apparatus added to a laser material processing head.
Meanwhile, various methods have been proposed for cleaning surface appearance after processing by previously coating a surface to be processed with a liquid material typified by oils and fats. For example, a coating method using a viscous material transparent to laser light is disclosed by Japanese Unexamined Patent Publication No. S52-112895, and an oil coating method by Japanese Unexamined Patent Publication No. S60-180686. Although material processing by melting with laser light is taken into account in these methods, the characteristics of coating substance are not described in these Publications. When any of oils and fats is used as coating substance, the transmittance of the coating substance relative to laser wavelength greatly affects surface appearance after processing (which is apparent from experimental research and study made by the present inventors). These Publications have no description suggesting knowledge relating to the present invention, and there is a problem that the suppression of dross deposition can not be realized with good reproducibility in forming holes on a metallic material with laser by the methods stated in the Publications.
With respect to the characteristics of coating substances, a coating method using one of oils and fats with a boiling point of 80° C. or higher is disclosed by Japanese Unexamined Patent Publication No. S58-110190, and the specification of the composition of coating material is disclosed by Japanese Unexamined Patent Publication No. H1-298113. In these disclosures, the former specifies only the boiling point of a coating material as the characteristic specification thereof, and has no disclosure on transmittance relative to the wavelength of the laser light used for hole forming. According to the experimental research done by the present inventors, there is a problem that dross generation can not be suppressed when oil or fat with large absorption is used even if its boiling point is 80° C. or higher. The latter discloses detailed composition and its basic concept is to specify a coating material that fulfills the function of enhancing the absorptivity relative to laser light, that is, of lowering the transmittance relative to laser light. In forming holes on a metallic material, a problem arises that the depositing property of dross is rather worsened if laser light absorption in a coating material is too large, thus failing to obtain an effective technique for dross suppression.